In general, a lithium secondary battery is required to have (1) high energy density (2) high output density (3) small self discharge ratio (4) economical aspect (5) high energy efficiency and (6) long cycle life.
As a secondary battery having such properties, a nonaqueous electrolyte battery utilizing an electric energy produced by the migration of lithium ions, i.e., so-called lithium secondary battery, is known to have high voltage and high energy density.
A lithium secondary battery comprising pure lithium as a negative electrode material can achieve high energy density of the negative electrode, whereas it suffers from a tendency to grow dendrite. An electrodeposition of metals generally proceeds in two steps of migration of metallic ions to the electrode surface (i.e. diffusion) and receipt and release of electrons at the electrode (i.e. electrode reaction). In the electrodeposition of pure lithium, the electrode reaction proceeds quickly and the speed of lithium diffusing toward the electrode surface determines the rate of electrodeposition. An electrodeposition wherein the rate is determined by such mass migration often suffers from the growth of dendrite. The dendrite is a branch-like crystal which grows quickly once formed, and penetrates a separator, as a result of which it causes short-circuit between the negative electrode and the positive electrode. Consequently, problems are caused which are undesirable in terms of safety, such as combustion, markedly short cycle life of the battery and poor cycle property.
On the other hand, a negative electrode composed of a lithium alloy comprising an intermetallic compound of Li and Al, Bi, Pb, Sn, In or the like is known to suppress deposition of lithium in a dendrite state, since the speed of absorption of lithium in the negative electrode becomes higher. However, this also poses problems in that the negative electrode becomes weak, so that it develops cracks through swelling and contraction in volume which occur along with the absorption and release of lithium, to ultimately result in pulverulent negative electrode, and that the electromotive force of the battery decreases due to a higher potential of the electrode than that possessed by the negative electrode composed of pure lithium.
In an attempt to provide a lithium secondary battery free from such problems of dendrite and the like, a lithium ion battery using a negative electrode prepared from a carbon material has been produced. However, the lithium ion battery of this construction is associated with a defect that the discharge capacity becomes small.
A variety of materials have been conventionally considered to be potential positive electrode active materials for lithium secondary batteries, and typically included therein are inorganic compounds capable of reversely inserting and eliminating lithium ions. As such inorganic compounds, chalcogen compounds such as oxides and sulfides have been mostly studied, and oxides having high potential, which are capable of achieving a high energy density of batteries, have been found to be particularly promising.
An example of the positive electrode active material comprising the above-mentioned oxide is LiCoO.sub.2. LiCoO.sub.2 is a 4 V class positive electrode active material reported by Mizushima et al. in 1980 (Mat. Res. Bull., Vol. 15, pp 783-789) and mainly used as a positive electrode active material for lithium secondary batteries which are being used as a main power source of portable equipments currently put to use. Again, a positive electrode active material using such LiCoO.sub.2 is inferior in the supply of materials since Co is expensive.
Accordingly, an object of the present invention is to solve the above-mentioned problems and provide a lithium secondary battery having great charge-discharge capacity and superior cycle property. Another object of the present invention is to provide a positive electrode active material for lithium secondary battery and a positive electrode material compound, which are free of the above-mentioned problems and which are economical, superior in suppliability, have great charge-discharge capacity and superior in cycle property.